The present invention relates to a composite substrate used for GaN epitaxial growth, especially an efficient composite substrate with a protective layer for preventing metal from diffusing.
In recent years, III/V nitride materials, mainly GaN, InGaN, and AlGaN, have received much attention as semiconductor materials. The III/V nitride materials have direct band gaps that can be continuously varied from 1.9 to 6.2 eV, excellent physical and chemical stability, and high saturation electron mobility. They have become the preferred materials for optoelectronic devices such as laser devices and light-emitting diodes.
However, to the present GaN-based semiconductor material device, due to a lack of GaN substrate, the epitaxial films of the GaN-based LED are mainly grown on the substrates such as sapphire substrates, SiC, Si substrate and so on. So far, epitaxial-growth technologies of GaN material series are basically based on highly mismatched hetero-epitaxy technologies. A hetero-epitaxial growth technology of sapphire substrate is the most widely used and protected. The main problems are that: firstly, the large lattice mismatch and thermal stress between the epitaxially grown GaN and the sapphire substrate can produce high concentration of dislocations of about 109 cm−2, which seriously degrades the quality of GaN crystal, and reduces illumination efficiency and the lifespan of LED. Secondly, because sapphire is an insulator with an electrical resistivity greater than 1011 Ωcm at room temperature, it is not suitable to be used for forming devices having vertical structures. Sapphire is usually only used to prepare N-type and P-type electrodes on the surface of the epitaxial layer, reducing effective lighting area, increasing the lithography and etching processes during the fabrication of the devices, and reducing the material utilization. Thirdly, sapphire has a poor thermal conductivity of about 0.25 W/cm K at 1000° C., which significantly affects performances of GaN-based devices, especially the large-area and high-power devices in which heat dissipation is required. Fourthly, sapphire has a high hardness and its lattice has a 30 degree angle relative to the lattice of GaN crystal, it is difficult to obtain a cleavage plane of the InGaN epitaxial layer to obtain a cavity surface during the fabrication of GaN-based Laser Diode (LD).
However, to the SiC substrate, it has lattice parameters closest to those of GaN and smaller lattice mismatch, but it also is hetero-epitaxy, and comprises misfit dislocations and thermal misfit dislocations. Moreover, SiC is expensive, making it unsuitable for many GaN-based optoelectronic devices (LED). In recently years, Si has also been studied as a substrate for the epitaxial growth of GaN. However, Si has a lattice mismatch to GaN even larger than sapphire/GaN, and Si has cubic crystalline lattice while GaN has a hexagonal crystalline lattice, which makes it difficult to support epitaxial growth of GaN material. The GaN layer grown on Si substrates faces serious problems such as cracking; the growth thickness usually cannot exceed 4 μm.
Therefore, to crystalline epitaxy, either the theory of epitaxial growth, or the development history of the semiconductor epitaxy technology, has proved that, homoepitaxy is an optimal selection. Recently, preparation technology of GaN mono-crystalline substrate has been developed, the appearance of GaN mono-crystalline substrate, makes GaN epitaxy return to homoepitaxy, and improves the quality of epitaxially grown GaN crystal. Moreover, the good thermal conductivity of the GaN crystals allows the GaN epitaxy substrate to be used in the formation of vertical structures for LED devices. The properties of the devices are improved under large current injections. However, the high cost of the GaN mono-crystalline substrate severely restricts its usage in LED devices. While a 2 inch wide high power LED epitaxial substrate is typically less than 100 dollars, the price for a 2 inch wide GaN mono-crystalline substrate currently can reach 2000 dollars.